Atomic weights shown in this table are 1977 values.
For long-lived isotope of an element that has no stable isotopes, standard atomic weights cannot be defined because of wide variaty of isotopic composition in nature.
For these elements, refer to "Atomic masses and abundances" table in ther following references.

- D.R.Linde ed., "CRC handbook of Chemistry and Physics", 76th ed., CRC Press, Boca Raton (1995)
- IUPAC Commission on Atomic Weights and Isotopic Abundances, Pure Appl. Chem., 63, 991 (1991)

Periodic Table of Elements